Sorry for my bad grammer in the last posts.
I buy Hengst, Mann, or Knect filters from Mercedesshop, dealer, or ebay - really whomever has the cheapest price at the time. These filters are readily available for $6-7 so I see no reason to buy anything else.
The main reasons oil is changed at 3000 miles (if I were using mineral oil, I'd do it at 2500) is because it starts to fall apart. The main reason to change a filter at 3000 is IFF you are using mineral oil, the filter is catching the oil fall apart. If you use synthetic, you dont have either problem.
So, if you want to be VERY conservative, just change your synthetic oil at 5000 miles along with your filter. Buy oil that is CI-4/CH-4 rated, or Euro spec B3/B4. Some oils are MB 229.1/229.3 rated which is even better.